Auto merge of #53851 - oli-obk:local_promotion, r=eddyb

Limit the promotion of const fns to the libstd and the `rustc_promotable` attribute

There are so many questions around promoting const fn calls... it seems saner to try to limit automatic promotion to const fns which were explicitly opted in for promotion.

I added the attribute to all public stable const fns that were already promotable (e.g. not Cell::new) in order to not cause any breakage

r? @eddyb

cc @nikomatsakis

src/test/run-pass/invalid_const_promotion.rs

@@ -14,7 +14,8 @@
 
 // compile-flags: -C debug_assertions=yes
 
-#![feature(const_fn, libc)]
+#![stable(feature = "rustc", since = "1.0.0")]
+#![feature(const_fn, libc, staged_api, rustc_attrs)]
 #![allow(const_err)]
 
 extern crate libc;
@@ -23,6 +24,8 @@ use std::env;
 use std::process::{Command, Stdio};
 
 // this will panic in debug mode and overflow in release mode
+#[stable(feature = "rustc", since = "1.0.0")]
+#[rustc_promotable]
 const fn bar() -> usize { 0 - 1 }
 
 fn foo() {

src/test/run-pass/issues/issue-49955-2.rs

@@ -13,13 +13,15 @@
 
 use std::cell::Cell;
 
+const FIVE: Cell<i32> = Cell::new(5);
+
 #[inline(never)]
 fn tuple_field() -> &'static u32 {
     // This test is MIR-borrowck-only because the old borrowck
     // doesn't agree that borrows of "frozen" (i.e. without any
     // interior mutability) fields of non-frozen temporaries,
     // should be promoted, while MIR promotion does promote them.
-    &(Cell::new(5), 42).1
+    &(FIVE, 42).1
 }
 
 fn main() {

src/test/ui/consts/const-call.rs

@@ -15,5 +15,4 @@ fn f(x: usize) -> usize {
 fn main() {
     let _ = [0; f(2)];
     //~^ ERROR calls in constants are limited to constant functions
-    //~| E0080
 }

src/test/ui/consts/const-call.stderr

@@ -4,13 +4,6 @@ error[E0015]: calls in constants are limited to constant functions, tuple struct
 LL |     let _ = [0; f(2)];
    |                 ^^^^
 
-error[E0080]: could not evaluate repeat length
-  --> $DIR/const-call.rs:16:17
-   |
-LL |     let _ = [0; f(2)];
-   |                 ^^^^ calling non-const fn `f`
+error: aborting due to previous error
 
-error: aborting due to 2 previous errors
-
-Some errors occurred: E0015, E0080.
-For more information about an error, try `rustc --explain E0015`.
+For more information about this error, try `rustc --explain E0015`.

src/test/ui/consts/const-eval/dont_promote_unstable_const_fn.nll.stderr

@@ -32,7 +32,7 @@ error[E0716]: temporary value dropped while borrowed
    |
 LL |     let x: &'static _ = &std::time::Duration::from_millis(42).subsec_millis();
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
-LL |     //~^ does not live long enough
+LL |     //~^ ERROR does not live long enough
 LL | }
    | - temporary value is freed at the end of this statement
    |

src/test/ui/consts/const-eval/dont_promote_unstable_const_fn.rs

@@ -31,5 +31,5 @@ fn a() {
 fn main() {
     let _: &'static u32 = &meh(); //~ ERROR does not live long enough
     let x: &'static _ = &std::time::Duration::from_millis(42).subsec_millis();
-    //~^ does not live long enough
+    //~^ ERROR does not live long enough
 }

src/test/ui/consts/const-eval/dont_promote_unstable_const_fn.stderr

@@ -32,7 +32,7 @@ error[E0597]: borrowed value does not live long enough
    |
 LL |     let x: &'static _ = &std::time::Duration::from_millis(42).subsec_millis();
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ temporary value does not live long enough
-LL |     //~^ does not live long enough
+LL |     //~^ ERROR does not live long enough
 LL | }
    | - temporary value only lives until here
    |

src/test/ui/consts/const-eval/issue-52443.stderr

@@ -1,51 +0,0 @@
-error[E0308]: mismatched types
-  --> $DIR/issue-52443.rs:12:10
-   |
-LL |     [(); & { loop { continue } } ]; //~ ERROR mismatched types
-   |          ^^^^^^^^^^^^^^^^^^^^^^^
-   |          |
-   |          expected usize, found reference
-   |          help: consider removing the borrow: `{ loop { continue } }`
-   |
-   = note: expected type `usize`
-              found type `&_`
-
-error[E0308]: mismatched types
-  --> $DIR/issue-52443.rs:13:17
-   |
-LL |     [(); loop { break }]; //~ ERROR mismatched types
-   |                 ^^^^^ expected (), found usize
-   |
-   = note: expected type `()`
-              found type `usize`
-
-error[E0019]: constant contains unimplemented expression type
-  --> $DIR/issue-52443.rs:14:11
-   |
-LL |     [(); {while true {break}; 0}]; //~ ERROR constant contains unimplemented expression type
-   |           ^^^^^^^^^^^^^^^^^^
-
-error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
-  --> $DIR/issue-52443.rs:15:21
-   |
-LL |     [(); { for _ in 0usize.. {}; 0}]; //~ ERROR calls in constants are limited to constant functions
-   |                     ^^^^^^^^
-
-error[E0019]: constant contains unimplemented expression type
-  --> $DIR/issue-52443.rs:15:21
-   |
-LL |     [(); { for _ in 0usize.. {}; 0}]; //~ ERROR calls in constants are limited to constant functions
-   |                     ^^^^^^^^
-
-error[E0080]: could not evaluate repeat length
-  --> $DIR/issue-52443.rs:15:10
-   |
-LL |     [(); { for _ in 0usize.. {}; 0}]; //~ ERROR calls in constants are limited to constant functions
-   |          ^^^^^^^^^^^--------^^^^^^^
-   |                     |
-   |                     calling non-const fn `<I as std::iter::IntoIterator><std::ops::RangeFrom<usize>>::into_iter`
-
-error: aborting due to 6 previous errors
-
-Some errors occurred: E0015, E0019, E0080, E0308.
-For more information about an error, try `rustc --explain E0015`.

src/test/ui/consts/const-eval/promoted_const_fn_fail.nll.stderr

@@ -0,0 +1,14 @@
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promoted_const_fn_fail.rs:30:27
+   |
+LL |     let x: &'static u8 = &(bar() + 1); //~ ERROR does not live long enough
+   |                           ^^^^^^^^^^^ creates a temporary which is freed while still in use
+...
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0716`.

src/test/ui/consts/const-eval/promoted_const_fn_fail.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-pass
-
 #![feature(const_fn, const_fn_union)]
 
 #![deny(const_err)]
@@ -29,10 +27,7 @@ const fn bar() -> u8 {
 }
 
 fn main() {
-    // FIXME(oli-obk): this should panic at runtime
-    // this will actually compile, but then
-    // abort at runtime (not panic, hard abort).
-    let x: &'static u8 = &(bar() + 1);
+    let x: &'static u8 = &(bar() + 1); //~ ERROR does not live long enough
     let y = *x;
     unreachable!();
 }

src/test/ui/consts/const-eval/promoted_const_fn_fail.stderr

@@ -0,0 +1,14 @@
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promoted_const_fn_fail.rs:30:27
+   |
+LL |     let x: &'static u8 = &(bar() + 1); //~ ERROR does not live long enough
+   |                           ^^^^^^^^^^^ temporary value does not live long enough
+...
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/consts/const-fn-error.stderr

@@ -1,43 +0,0 @@
-error[E0658]: let bindings in constant functions are unstable (see issue #48821)
-  --> $DIR/const-fn-error.rs:16:19
-   |
-LL |     let mut sum = 0;
-   |                   ^
-   |
-   = help: add #![feature(const_let)] to the crate attributes to enable
-
-error[E0658]: statements in constant functions are unstable (see issue #48821)
-  --> $DIR/const-fn-error.rs:16:19
-   |
-LL |     let mut sum = 0;
-   |                   ^
-   |
-   = help: add #![feature(const_let)] to the crate attributes to enable
-
-error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-  --> $DIR/const-fn-error.rs:19:14
-   |
-LL |     for i in 0..x {
-   |              ^^^^
-
-error[E0019]: constant function contains unimplemented expression type
-  --> $DIR/const-fn-error.rs:19:14
-   |
-LL |     for i in 0..x {
-   |              ^^^^
-
-error[E0080]: could not evaluate constant expression
-  --> $DIR/const-fn-error.rs:29:13
-   |
-LL |     for i in 0..x {
-   |              ---- calling non-const fn `<I as std::iter::IntoIterator><std::ops::Range<usize>>::into_iter`
-...
-LL |     let a : [i32; f(X)]; //~ ERROR E0080
-   |             ^^^^^^----^
-   |                   |
-   |                   inside call to `f`
-
-error: aborting due to 5 previous errors
-
-Some errors occurred: E0015, E0019, E0080, E0658.
-For more information about an error, try `rustc --explain E0015`.

src/test/ui/consts/min_const_fn/promotion.nll.stderr

@@ -0,0 +1,68 @@
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promotion.rs:13:27
+   |
+LL |     let x: &'static () = &foo1(); //~ ERROR does not live long enough
+   |                           ^^^^^^ creates a temporary which is freed while still in use
+...
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promotion.rs:14:28
+   |
+LL |     let y: &'static i32 = &foo2(42); //~ ERROR does not live long enough
+   |                            ^^^^^^^^ creates a temporary which is freed while still in use
+...
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promotion.rs:15:28
+   |
+LL |     let z: &'static i32 = &foo3(); //~ ERROR does not live long enough
+   |                            ^^^^^^ creates a temporary which is freed while still in use
+...
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promotion.rs:16:34
+   |
+LL |     let a: &'static Cell<i32> = &foo4();  //~ ERROR does not live long enough
+   |                                  ^^^^^^ creates a temporary which is freed while still in use
+...
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promotion.rs:17:42
+   |
+LL |     let a: &'static Option<Cell<i32>> = &foo5(); //~ ERROR does not live long enough
+   |                                          ^^^^^^ creates a temporary which is freed while still in use
+LL |     let a: &'static Option<Cell<i32>> = &foo6(); //~ ERROR does not live long enough
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promotion.rs:18:42
+   |
+LL |     let a: &'static Option<Cell<i32>> = &foo6(); //~ ERROR does not live long enough
+   |                                          ^^^^^^ creates a temporary which is freed while still in use
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0716`.

src/test/ui/consts/min_const_fn/promotion.rs

@@ -0,0 +1,19 @@
+#![feature(min_const_fn)]
+
+use std::cell::Cell;
+
+const fn foo1() {}
+const fn foo2(x: i32) -> i32 { x }
+const fn foo3() -> i32 { 42 }
+const fn foo4() -> Cell<i32> { Cell::new(42) }
+const fn foo5() -> Option<Cell<i32>> { Some(Cell::new(42)) }
+const fn foo6() -> Option<Cell<i32>> { None }
+
+fn main() {
+    let x: &'static () = &foo1(); //~ ERROR does not live long enough
+    let y: &'static i32 = &foo2(42); //~ ERROR does not live long enough
+    let z: &'static i32 = &foo3(); //~ ERROR does not live long enough
+    let a: &'static Cell<i32> = &foo4();  //~ ERROR does not live long enough
+    let a: &'static Option<Cell<i32>> = &foo5(); //~ ERROR does not live long enough
+    let a: &'static Option<Cell<i32>> = &foo6(); //~ ERROR does not live long enough
+}

src/test/ui/consts/min_const_fn/promotion.stderr

@@ -0,0 +1,68 @@
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promotion.rs:13:27
+   |
+LL |     let x: &'static () = &foo1(); //~ ERROR does not live long enough
+   |                           ^^^^^^ temporary value does not live long enough
+...
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promotion.rs:14:28
+   |
+LL |     let y: &'static i32 = &foo2(42); //~ ERROR does not live long enough
+   |                            ^^^^^^^^ temporary value does not live long enough
+...
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promotion.rs:15:28
+   |
+LL |     let z: &'static i32 = &foo3(); //~ ERROR does not live long enough
+   |                            ^^^^^^ temporary value does not live long enough
+...
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promotion.rs:16:34
+   |
+LL |     let a: &'static Cell<i32> = &foo4();  //~ ERROR does not live long enough
+   |                                  ^^^^^^ temporary value does not live long enough
+...
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promotion.rs:17:42
+   |
+LL |     let a: &'static Option<Cell<i32>> = &foo5(); //~ ERROR does not live long enough
+   |                                          ^^^^^^ temporary value does not live long enough
+LL |     let a: &'static Option<Cell<i32>> = &foo6(); //~ ERROR does not live long enough
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promotion.rs:18:42
+   |
+LL |     let a: &'static Option<Cell<i32>> = &foo6(); //~ ERROR does not live long enough
+   |                                          ^^^^^^ temporary value does not live long enough
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/issues/issue-39559-2.rs

@@ -23,8 +23,6 @@ impl Dim for Dim3 {
 fn main() {
     let array: [usize; Dim3::dim()]
     //~^ ERROR E0015
-    //~| ERROR E0080
         = [0; Dim3::dim()];
         //~^ ERROR E0015
-        //~| ERROR E0080
 }

src/test/ui/issues/issue-39559-2.stderr

@@ -5,26 +5,11 @@ LL |     let array: [usize; Dim3::dim()]
    |                        ^^^^^^^^^^^
 
 error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
-  --> $DIR/issue-39559-2.rs:27:15
+  --> $DIR/issue-39559-2.rs:26:15
    |
 LL |         = [0; Dim3::dim()];
    |               ^^^^^^^^^^^
 
-error[E0080]: could not evaluate repeat length
-  --> $DIR/issue-39559-2.rs:27:15
-   |
-LL |         = [0; Dim3::dim()];
-   |               ^^^^^^^^^^^ calling non-const fn `<Dim3 as Dim>::dim`
+error: aborting due to 2 previous errors
 
-error[E0080]: could not evaluate constant expression
-  --> $DIR/issue-39559-2.rs:24:16
-   |
-LL |     let array: [usize; Dim3::dim()]
-   |                ^^^^^^^^-----------^
-   |                        |
-   |                        calling non-const fn `<Dim3 as Dim>::dim`
-
-error: aborting due to 4 previous errors
-
-Some errors occurred: E0015, E0080.
-For more information about an error, try `rustc --explain E0015`.
+For more information about this error, try `rustc --explain E0015`.

src/test/ui/issues/issue-43105.rs

@@ -16,7 +16,6 @@ const NUM: u8 = xyz();
 fn main() {
     match 1 {
         NUM => unimplemented!(),
-        //~^ ERROR could not evaluate constant pattern
         _ => unimplemented!(),
     }
 }

src/test/ui/issues/issue-43105.stderr

@@ -4,16 +4,6 @@ error[E0015]: calls in constants are limited to constant functions, tuple struct
 LL | const NUM: u8 = xyz();
    |                 ^^^^^
 
-error[E0080]: could not evaluate constant pattern
-  --> $DIR/issue-43105.rs:18:9
-   |
-LL | const NUM: u8 = xyz();
-   |                 ----- calling non-const fn `xyz`
-...
-LL |         NUM => unimplemented!(),
-   |         ^^^
+error: aborting due to previous error
 
-error: aborting due to 2 previous errors
-
-Some errors occurred: E0015, E0080.
-For more information about an error, try `rustc --explain E0015`.
+For more information about this error, try `rustc --explain E0015`.

src/test/ui/rustc-args-required-const.rs

@@ -27,7 +27,8 @@ const fn baz() -> i32 {
 fn main() {
     foo(2);
     foo(2 + 3);
-    foo(baz());
+    const BAZ: i32 = baz();
+    foo(BAZ);
     let a = 4;
     foo(A);
     foo(a); //~ ERROR: argument 1 is required to be a constant

src/test/ui/rustc-args-required-const.stderr

@@ -1,11 +1,11 @@
 error: argument 1 is required to be a constant
-  --> $DIR/rustc-args-required-const.rs:33:5
+  --> $DIR/rustc-args-required-const.rs:34:5
    |
 LL |     foo(a); //~ ERROR: argument 1 is required to be a constant
    |     ^^^^^^
 
 error: argument 2 is required to be a constant
-  --> $DIR/rustc-args-required-const.rs:35:5
+  --> $DIR/rustc-args-required-const.rs:36:5
    |
 LL |     bar(a, a); //~ ERROR: argument 2 is required to be a constant
    |     ^^^^^^^^^